4.1 Introduction

The biological process of denitrification involves the reduction of nitrate nitrogen, N03", to a gaseous nitrogen species. The gaseous product is primarily nitrogen gas, N2, but may also be nitrous oxide, N20, or nitric oxide, NO. Gaseous nitrogen is not readily available for biological growth, thus denitrification converts nitrogen to a harmless form which has no significant effect on the environment.

Some confusion has arisen in the terminology used in the literature. The process has been termed anaerobic denitrification. The principal biochemical pathways, however, are not anaerobic, but merely minor modifications of aerobic biochemical pathways. The term anoxic denitrification is therefore preferable, as it describes the environmental condition involving the absence of oxygen, without implying the nature of the biochemical pathways.

Denitrification is of interest because:

1. It is a major mechanism for loss of fertilizer nitrogen in agriculture, resulting in a decreased efficiency of the fertilizer.

2. It is of great potential application in the removal of nitrogen from high-nitrogen waste materials such as animal residues.

3. Many factors affect the accumulation of denitrification intermediates, such as N20, but only very few attempts have been made to develop a unifying explanation of the different intermediates.

4. Denitrification is the mechanism by which the global nitrogen cycle is balanced.

5. Most ground water resources of the world are facing a major nitrate contamination, which may result in infant methemoglobi.

6. It is a method for the removal of nitrogen from waste water.

The contribution of waste treatment systems to atmosheric N20 is of some concern, because N20 is involved in the stratospheric reactions, which result in the depletion of ozone, but little information is available. It is noteworthy, however, that fermentation, waste water acclimated to or supplemented with nitrate, released small quantities of N20 during denitrification, whereas the waste water adapted to or supplied with nitrite, produced none.

Nitrate contamination of ground water resources is becoming an ever increasing problem. Because of the adverse effects on health associated with nitrate in drinking water, and the concerns regarding diminishing water quality, the interest in nitrate removal technologies increases.

The drinking-water standard set by the U.S. Environmental Protection Agency (EPA), for nitrate is 10 mg/l as nitrate-nitrogen. The European Economic Community has a standard of 50 mg/l as nitrate (11,3 mg/l nitrate-nitrogen).

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